Entity Relationships

@OneToMany(mappedBy = "user")
private List<Order> orders;

@ManyToOne
private User user;

import jakarta.persistence.*;
import java.util.*;

@Entity
class User {

  @Id
  private int id;
  private String name;

  @OneToMany(mappedBy = "user")
  private List<Order> orders;
}

@Entity
class Order {

  @Id
  private int id;
  private String product;

  @ManyToOne
  @JoinColumn(name = "user_id")
  private User user;
}

public class Main {
  public static void main(String[] args) {
    System.out.println("Entity Relationships Example");
  }
}

// Output:
// Entity Relationships Example

• 1 Define relationships between database tables.
• 2 Implemented using JPA annotations.
• 3 Represent real-world associations.
• 4 Used in ORM mapping.

• 1 One-to-One (@OneToOne)
• 2 One-to-Many (@OneToMany)
• 3 Many-to-One (@ManyToOne)
• 4 Many-to-Many (@ManyToMany)

• 1 User → Orders (One-to-Many)
• 2 Employee → Department (Many-to-One)
• 3 Student → Courses (Many-to-Many)
• 4 User → Profile (One-to-One)

• 1 Represent real-world data structure.
• 2 Enable relational database design.
• 3 Improve data consistency.
• 4 Support complex queries.